In the field of energy production technology, methods for better and safer fossil fuel utilization occupy an important position. The "energy crisis" resulting from man's ever increasing demands for energy places a premium on finding ways of economically but cleanly consuming fossil fuels. Because of the immense coal reserves which exist on earth, large quantities of coal are used in the production of energy. Methods of efficiently burning or gasifying coal with reduced harmful emissions to the atmosphere are therefore of major importance.
Sulfur in coal is a major source of polluting emissions, in the form of SO.sub.2 when coal is burned and H.sub.2 S when coal is gasified. The use of calcium in sulfur-containing coal as a means of preventing the release of gaseous sulfur compounds during combustion has been widely investigated. For example, see the article by Freund et al. entitled "The Sulfur Retention of Calcium-Containing Coal during Partial Oxidation," in Proceedings of the International Conference on Coal Science, held Sept. 7-9, 1981 in Dusseldorf, Germany; the article by Freund and Lyon entitled "The Sulfur Retention of Calcium-Containing Coal during Fuel-Rich Combustion," in the journal COMBUSTION AND FLAME, Volume 45, pp. 191-203, published in 1982; and the article by Chang et al. in the journal FUEL, Volume 65, at page 75, published in 1986. The methods for adding calcium to coal which have been proposed have included limestone injection and ion exchange between the coal and a calcium solution at a suitable pH.
More recently Sharma et al. have proposed an alternate method of treating coal with calcium, described in the article "Calcium pretreatment of coal for sulphur emissions control in combustion," in the journal FUEL, Volume 66, pp. 207-209, published in February of 1987. The proposed process utilizes a liquid-phase ionic reaction between Ca.sup.2+, CO.sub.2, and H.sub.2 O to deposit submicron-size CaCO.sub.3 particles in coal pores. An aqueous-phase reaction between CO.sub.2 (contained within the coal's pore network) and calcium acetate takes place in which calcium replenishment is provided by calcium hydroxide. In this method proposed by Sharma et al. the inclusion of CO.sub.2 within the coal is achieved by first evacuating the coal (by means of a vacuum pump) and then adsorbing CO.sub.2 onto the coal by contact with CO.sub.2 at or near a pressure of one atmosphere. The individual and overall reactions in the impregnation process are described as: ##EQU1##
The economics of the impregnation process is favorably influenced by the fact that the solubilizing anion, acetate, is not consumed in the process. However, the proposed process involves evacuating the coal and then adsorbing CO.sub.2 on it, a process which is not economically practical. The step in which coal is evacuated prior to CO.sub.2 adsorption will be costly considering the large amounts of coal handled in industry.
If a method could be found which makes use of the chemical processes suggested by Sharma et al., but which is simpler and economically feasible, it would be a great advance in the field of coal combustion technology, since it would allow for very efficient and clean utilization of coal as an energy source.